1. Field of the Invention
The present invention relates to a brushless DC fan driver, and more particularly, to a brushless DC fan driver having an over voltage prevention circuit, a lock protection circuit and a fan tachometer output to provide a reliable driving capability.
2. Description of Related Art
U.S. Pat. No. 5,289,089 granted to Aoki disclosed a fan motor drive system which employs a fan motor drive circuit to drive a single phase motor (10). The fan motor drive circuit has an amplifier (2) to receive output signals from a Hall element (1) for determining the phase of the rotor in a single phase motor (10) and output the same after amplification. A drive signal generating circuit (3) receives the amplified signal from the amplifier (2), generates first and second pulses (4a , 5a)having a duty cycle of about 50% where the phases differ by about 180 degrees from each other and outputs the same. A first drive circuit (11) is connected to the first terminal (6) of the single phase motor (10), and a second drive circuit (12) is connected to the second terminal (6) of the single phase motor (10), each of the first and second drive circuits (11, 12) includes a first transistor which supplies a current to the single phase motor (10), a second transistor which draws a current from the single phase motor (10) and a diode which bypasses the current flowing out of the single phase motor (10) toward a power source line side.
When the above fan motor drive system is operating, overshoot voltages may be generated on the outputs of the fan motor drive circuit, which will cause gradual damage to the fan motor drive system. Therefore, two diodes (D1,D2) are used to bypass the overshoot voltage to the power source line. However, in some applications, the power source has to be connected in series with a diode to prevent permanent damage caused by faulty connection between VCC and ground. This makes diodes protection useless.
Further, there is no protection for the above fan motor drive system when the fan motor (10) is locked up. The rotation speed of the fan motor (10) is also unknown so that the user is not aware of the operation condition of the fan motor (10) and the rotation of the fan motor (10) can not be properly controlled. Therefore, there is a need for the above fan motor drive circuit to be improved.